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 Multiplexed, Super-long Larson Scanner?
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By: Anonymous: GJ () on Thursday, October 14 2010 @ 09:45 AM PDT  
Anonymous: GJ

If you're going to expand the Larson Scanner, why not reconsider using a microcontroller at all? It makes sense when you don't have that many pins, but once you start adding more LED's, you're forced to use multiple microcontrollers, and that gets expensive pretty quick. Instead, it'd be cheaper to chain together multiple '4017 chips together to scan across an arbitrarily long array of LED's, (and back, just by feeding the outputs of of the next half back into the same LED's.) You can chain the 4017's like this: image
and never have to bother with any microcontrollers. All you'd need is an IMC7555, (lower power 555), a pile of 74HC4017's, and 1 74HC08 (it's a quad-AND gate) for every four decade counters you want to use. I understood the utility of switching to an ATTiny when you just had 5 LED's to worry about, but it seems to me once you start chaining enough LED's together you end up wasting money with microcontrollers when TTL logic can do the job for you.





       
   
By: squall_line (offline) on Thursday, October 14 2010 @ 10:03 AM PDT  
squall_line

Quote by: GJ

If you're going to expand the Larson Scanner, why not reconsider using a microcontroller at all? It makes sense when you don't have that many pins, but once you start adding more LED's, you're forced to use multiple microcontrollers, and that gets expensive pretty quick.

Instead, it'd be cheaper to chain together multiple '4017 chips together to scan across an arbitrarily long array of LED's, (and back, just by feeding the outputs of of the next half back into the same LED's.)

You can chain the 4017's like this:

and never have to bother with any microcontrollers. All you'd need is an IMC7555, (lower power 555), a pile of 74HC4017's, and 1 74HC08 (it's a quad-AND gate) for every four decade counters you want to use.

I understood the utility of switching to an ATTiny when you just had 5 LED's to worry about, but it seems to me once you start chaining enough LED's together you end up wasting money with microcontrollers when TTL logic can do the job for you.



Yikes! Could you edit your post to pare down the size of that image? It's totally FUBARed the layout of the thread...

The original design of the circuit used 4017s and 555s: http://www.evilmadscientist.com/article.php/CylonOLantern

And, from the article about the ATtiny-based Larson Scanner (http://www.evilmadscientist.com/article.php/larsonkit), one of the first mentions is this:

"And while it's been popular, we've always had some nagging reservations about it, and in particular its battery life. This year, we decided to do something about it and made a much better version of the Larson Scanner"


Personally, I went the route I went with the expandable scanner because it was a hack/mod of the original project, using the original parts. Not necessarily cost-effective, for sure, but the purpose of the expansion project was to allow people to use the original form-factor, with PCBs that are already built and parts that are already purpose-built for the task. In contrast, one could potentially create a bigger scanner with an ATmegaxx8 chip, to run more outputs at a time, too.

Also, as mentioned earlier in this thread, I had considered using multi-plexing to expand the project, which I might still consider doing, but the boards themselves lend themselves more towards a linking-type of expansion than a multi-plexing expansion. A multi-plexed method would reduce the chip count back to one, but would require a significant overhaul in the PCB design (IMHO), and some pretty gnarly wiring to run more sets of LEDs off-board.

As it is, the form-factor of the boards is perfect for what I eventually intend to do with them, so I'm sticking with my method for now...


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By: Anonymous: GJ () on Thursday, October 14 2010 @ 10:17 AM PDT  
Anonymous: GJ

Yeah, I guess I can see the virtue of modifying nothing but firmware and a couple of wires instead of having to build an entirely new circuit. It's just that I preferred the old design - It was minimalist.

All you really for a larson scanner is a clock and a counter, and implementing it with a 555 and the 74HC4017 do precisely that and pretty much nothing else. I find that uplifting and elegant. Plus using the low power versions means you're not really wasting power any more. If anything, I'd guess that they'd consume less juice than a bona fide micro, though I have no evidence to suggest that one way or the other.

Plus, ya gotta admit: The way they figured out how to chain 4017's is just so clever.





       
   
By: squall_line (offline) on Thursday, October 14 2010 @ 10:31 AM PDT  
squall_line

Quote by: GJ

Yeah, I guess I can see the virtue of modifying nothing but firmware and a couple of wires instead of having to build an entirely new circuit. It's just that I preferred the old design - It was minimalist.

All you really for a larson scanner is a clock and a counter, and implementing it with a 555 and the 74HC4017 do precisely that and pretty much nothing else. I find that uplifting and elegant. Plus using the low power versions means you're not really wasting power any more. If anything, I'd guess that they'd consume less juice than a bona fide micro, though I have no evidence to suggest that one way or the other.

Plus, ya gotta admit: The way they figured out how to chain 4017's is just so clever.



I'm not sure how much more minimalist a PCB, a chip, some resistors, and some LEDs can be? The original Cylon-o-Lantern had transistors, capacitors, diodes... None of which are necessary for the Larson Scanner.

And, the LS can run off of a pair of AA batteries, for hours upon hours at a time. A 9V battery that lasts only a couple of hours is a power-hog by comparison, IMHO.

Also, as you said, all it takes to modify the cycle order, number of lit LEDs, brightness, speed, etc. of the Larson Scanner is a firmware update. Or, if the code is written the right way, a button press on the board itself.


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By: Windell (offline) on Thursday, October 14 2010 @ 10:44 AM PDT  
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In a word, the reason is "simplicity."

Simplicity is a tough concept; there are different types. The original version does not need programming. You can build it right from off the shelf parts. Its parts cost is lower. Those are important advantages.

But the component count is higher, the battery life is lower, and it supports fewer LEDs. It takes much longer to assemble. The digital version is much faster to assemble (For me personally, it's 10 minutes versus 30, assuming no PCB-- or about 5 minutes with the PCB!), its parts count is lower, it has 1.5 times the number of LEDs, and it's battery efficient.

Setting aside number of LEDs and battery life, the big win is this: we've moved the complexity of the circuit from the wiring to the program. For doing multiple scanners, I'd definitely want to do the digital version. The savings in terms of assembly time adds up quickly. (Also, at 9 LEDs versus 6, you've got a head start already.) For repurposing, hacking, changing the response function and so on, the digital is arguably more flexible. Making the "discrete logic" version into a POV display is a lot harder, for example.

In terms of cost, it's a toss-up as well, depending what you're doing. Suppose that you want to light a pumpkin for a week. If you start from scratch-- a bare '2313 or '4017 --that's the cheapest way. The digital version will break even with the other by the 2nd or 3rd 9V battery, well under a week. For me, the amount of time required is the most important factor: I value my free time very highly, so it's worth a little extra cost if it saves a lot of time.


Back when we first released the "discrete logic" version, people gave us a hard time for it-- "Why don't you learn to use a microcontroller?" (Ouch!) Naturally, when we now talk about the digital version, people give us a hard time for it-- "Why don't you just use discrete logic?" But as I hope you can see, there are real advantages to both, and that's why we've got both versions out there now.


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http://www.evilmadscientist.com/

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